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-formic acid and its deuterated isotopologs: Combining theory and experiment to extend the vibrational database
No full textA path from gas to liquid: Development of model Hamiltonians for condensed phase local mode IR spectra via benzene (Bz) and Bz2
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Previous issue date: 2019-06-18Condensed-phase systems represent a vast swath of chemical problems that are of biological and atmospheric interest. Force fields for simulations of these liquid phases need to be able to model intermolecular interactions accurately. Moreover, reliable potentials for spectroscopic calculations need to reproduce site energies and molecular properties, as well as predict fluctuations in molecular coupling appropriately. A local mode approach to the calculation of IR spectra of condensed phase systems of CH stretches will be taken. This is justified because solutions of only CH stretches tend to exhibit less correlated network behavior than solutions of mixed CH/XH (X=O,N) stretch solvents due to weaker intermolecular forces. In an effort to understand solvent effects in the condensed phase, previously developed model gas-phase Hamiltonians will be extended to describe liquid CH stretch systems. As a first step, benzene will be investigated due to its high degree of symmetry and availability of force fields that describe its intermolecular interactions. Benzene represents a technical challenge for vibrational spectroscopy due to the presence of a Fermi triad in its CH stretch IR spectrum. By incorporating these couplings into the model Hamiltonian, it will be possible to consider solvent effects on the IR spectrum of the noncovalently bound benzene dimer. Extension of the dimer model to the condensed phase will allow for discussion of other relevant spectra of benzene, especially its vibrational sum-frequency generation (VSFG) spectrum
A zero-order picture of the infrared spectrum for the methoxy radical: assignment of states
Full text linkThe ground vibrations of the methoxy radical have
intrigued both experimentalists and theorists alike due to the presence of a conical
intersection at the C molecular geometry. This conical intersection causes
methoxy's vibrational spectrum to be strongly influenced by Jahn-Teller vibronic coupling which
leads to large amplitude vibrations and extensive mixing of the two lowest
electronic states. This coupling combined with spin-orbit and Fermi couplings
greatly complicates the assignments of states. \medskip
Using the potential force field and calculated spectra of
Nagesh and Sibert, we assign quantum numbers to the infrared spectrum. When the zero-order states are the diabatic normal mode states, there is sufficient mode mixing that the normal mode quantum numbers are poor labels for the final states. We define a series of zero-order Hamiltonians which include additional coupling elements beyond the normal mode picture but still allow for the assignment of Jahn-Teller quantum numbers. In methoxy, the two lowest frequency {\it e} modes, the bend () and the rock (), are the modes with the strongest Jahn-Teller coupling. In general, a zero-order Hamiltonian which includes first-order Jahn-Teller coupling in is sufficient for most states of interest. Working in a representation which includes first-order Jahn-Teller coupling in , we identify states in which additional coupling elements must be includedthese couplings include first-order Jahn-Teller coupling in , higher order Jahn-Teller coupling in and , and, in the dueterated case, Jahn-Teller coupling which is modulated by the corresponding {\it a} modes.
\medskip
\begin{list}{}{}\item[] Nagesh, J.Sibert, E.~L. {\em J. Phys. Chem. A} {\bf 2012},
{\em 116}, 3846--3855.
\item[] Lee, Y.F.Chou, W.T.Johnson, B.A.Tabor, D.P.Sibert, E.L.Lee, Y.P. {\em J. Mol. Spectrosc.} {\bf 2015}, {\em 310}, 57-67.
\item[] Barckholtz, T.~A.Miller, T.~A. {\em Int. Revs. in Phys. Chem.} {\bf
1998}, {\em 17}, 435--524.
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Previous issue date: 2016-06-2
Assigning States In The Jahn-teller Coupled Infrared Spectra Of Ch3o And Cd3o
Full text link\hspace*{1cm} The ground vibrations of the methoxy radical have
intrigued both experimentalists and theorists alike due to the presence of a conical
intersection at the C molecular geometry. This conical intersection causes
methoxy's vibrational spectrum to be strongly influenced by Jahn-Teller coupling,
this leading to large amplitude vibrations and extensive mixing of the two lowest
electronic states. This coupling combined with spin-orbit and Fermi couplings
greatly complicates the assignments of states. In this talk we describe our efforts
to assign the states of both CHO and CDO.
\hspace*{1cm} Using the potential energy force field and calculated spectra of
Nagesh and Sibert as a starting point, vibrational mixing is considered using
various zero-order representations. When the zero-order states are the diabatic
normal mode states, there is sufficient mode mixing that the normal mode quantum
numbers are no longer good labels. The mixing of the zero-order states can be
reduced by including additional terms in the zero-order Hamiltonian, . We
consider the choice of including the first order Jahn-Teller coupling between one of
the three degenerate normal modes. As the rocking motion has the largest
Jahn-Teller coupling, this is the coupling that is included in . Although the
normal mode quantum numbers of the rocking basis functions are no longer good
quantum numbers, due to the Jahn-Teller induced vibronic mixing, the zero-order
states can be labeled with the linear Jahn-Teller quantum numbers. This work
extends these ideas by considering an that includes linear Jahn-Teller
coupling between {\it two} sets of degenerate vibrations. Plots of the resulting
zero-order states are presented, and the spectral transitions recently
observed for both CHO and CDO in a -H matrix are assigned using
these basis functions. The extent of state-mixing found for the full Hamiltonian
for various choices of is illustrated via the use of correlation diagrams
obtained by plotting the eigenvalues of as a function of
where varies from zero to one.
\begin{list}{}{}\item[] Nagesh, J.; Sibert, E.~L. {\em J. Phys. Chem. A} {\bf 2012},
{\em 116}, 3846--3855.
\item[] Barckholtz, T.~A.; Miller, T.~A. {\em Int. Revs. in Phys. Chem.} {\bf
1998}, {\em 17}, 435--524.
\item[] Yu-Fang Lee, Wei-Te Chou, and Yuan-Pern Lee (private communication).
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Previous issue date: 2014-06-1
A zero-order picture of the infrared spectra of CH3O and CD3O: Assignment of states
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Previous issue date: 6Experimentalists and theorists alike have been intrigued by the infrared spectra of the methoxy radical; due to the presence of a conical_x000d_
intersection at the C molecular geometry,_x000d_
methoxy's IR spectrum is strongly influenced by Jahn-Teller vibronic coupling which_x000d_
leads to large amplitude vibrations and extensive mixing of the two lowest_x000d_
electronic states. This radical's complex IR spectra, which also contains moderate mixing from spin-orbit and Fermi couplings, serves as_x000d_
an important test for models which seek to understand complex molecular vibrations._x000d_
_x000d_
The assignment of the IR spectra in methoxy, and its partially and fully deuterated analogues, is considered._x000d_
All vibronic states below 2575 cm in CHO and 2035 cm in CDO_x000d_
are assigned. _x000d_
The mixing between the zero-order normal modes complicates the assignment using this representation._x000d_
Alternative zero-order representations, that include specific Jahn-Teller couplings, are explored and used to create_x000d_
definitive assignments for the low lying vibronic states. In many instances it is possible to plot the wavefunctions on which the assignments are based. The plots also enable one to visualize the conical seam_x000d_
and its effect on the wavefunctions. The first and second order Jahn-Teller coupling in the rocking motion dominates_x000d_
the spectral features in CHO, while first order and modulated first order couplings dominate the spectral features in CDO._x000d_
The methods described here are general and can be applied to other Jahn-Teller systems._x000d
Extending the local mode Hamiltonian into the condensed phase: Using vibrational sum frequency generation to study the benzene-air interface
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Previous issue date: 6Surfaces and interfaces play an important role in understanding many chemical process; they also contain molecular configurations and vibrations that are unique compared to those seen in the bulk and gas phases. Sum frequency generated (SFG) vibrational spectroscopy provides an incredibly detailed picture of these interfaces. In particular, the CH stretch region of the spectrum contains an extensive degree of information about the molecular vibrations and arrangements at the surface or interface. The presence of a strong bandwidth SFG signal for the benzene/air interface has generated controversy since it was discovered; since benzene is centrosymmetric, no SFG signal is expected. _x000d_
It has been hypothesized that this signal is primarily a result of bulk contributions that results from electric quadrupole transitions. Our work focuses on testing this conclusion by calculating a theoretical VSF spectrum from pure surface contributions using a mixed quantum/classical local mode Hamiltonian._x000d_
_x000d_
We take as a starting point our local mode CH/OH stretch Hamiltonian, that was previously used to study alkylbenzenes, benzene-(HO), and DPOE-water clusters, and extend it to the condensed phase by including shifts in the intensities and frequencies as a function of the environment. _x000d_
This environment is modeled using a SAPT-based force-field that accurately reproduces the quadrupole for the benzene dimer._x000d_
A series of independent time-dependent trajectories are used to obtain an ensemble of surface configurations and calculate the appropriate correlation functions. These correlations functions allow us to determine the origins of the VSF signal. Our talk will focus on the challenges of extending our local mode Hamiltonian into the condensed phase
Gaussian processes for spectroscopically accurate potential energy surfaces of benzene dimer
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Previous issue date: 23Gaussian process regression (GPR) is a robust method for fitting functions due to the flexible ways in which covariance between calculated \textit{ab initio} single points can be described and the relatively low amount of these training points required to achieve convergence. Increasing the complexity of the covariance (kernel) functions will systematically improve the fidelity of any given fit for functions of up to several dozen variables before the GPR becomes too computationally expensive to perform. This talk will focus on the development of a GPR algorithm to describe a 5-dimensional potential energy surface for the benzene dimer, and subsequent applications of the learned surface to spectroscopic calculations. The high symmetry of benzene allows us to decrease the number of training points even further than previously required while still maintaining faithful representation of the underlying PES. Through systematic engineering of a sufficient kernel function and both stochastic and strategic selection of training points in the conformational space of the benzene dimer, we will have access to a complete map of the necessary molecular parameters to predict accurate IR, Raman, and VSFG spectra for benzene from the gas to the condensed phase
JAHN-TELLER COUPLING IN THE METHOXY RADICAL: INSIGHTS INTO THE INFRARED SPECTRUM OF MOLECULES WITH VIBRONIC COUPLING
Full text linkhspace*{1cm} The ground vibrations of the methoxy radical have
intrigued both experimentalists and theorists alike due to the presence of a conical
intersection at the molecular geometry. This conical intersection causes
methoxy's vibrational spectrum to be strongly influenced by Jahn-Teller coupling which
leads to large amplitude vibrations and extensive mixing of the two lowest
electronic states. The spectrum is further complicated due to spin-orbit and Fermi couplings. The standard diabatic normal mode quantum numbers are poor labels due to this vibronic mixing. medskip
hspace*{1cm} Using the potential energy force field and calculated spectra of the methoxy radical by
Nagesh and Sibert as a starting point, we look to develop a method for assigning states to a spectrum with vibronic coupling. We simplify the analysis by considering only the lowest two {em e} modes of methoxy (the rock and the bend). When we include first-order Jahn-Teller coupling between these two modes in a new zero-order Hamiltonian, we are able to use an expanded version of the linear Jahn-Teller quantum numbers to assign the states. This zeroth order representation is nontrivial; therefore, we study the properties of its eigenstates using correlation diagrams with respect to the strength of the Jahn-Teller coupling constant.
medskip
begin{list}{}{}item[] Nagesh, J.; Sibert, E.~L. {em J. Phys. Chem. A} {bf 2012},
{em 116}, 3846--3855.
item[] Barckholtz, T.~A.; Miller, T.~A. {em Int. Revs. in Phys. Chem.} {bf
1998}, {em 17}, 435--524.
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Previous issue date: 2
A FULLY AB INITIO APPROACH TO THE EVALUATION OF VIBRATIONAL SPECTRA USING GAUSSIAN BASIS SETS
Full text linkWe have developed a black-box method to evaluate vibrational spectra using only the information provided by semi-classical ab initio molecular dynamics simulations. Using a distributed Gaussian basis set centered at the points sampled by an AIMD trajectories and a local harmonic approximation to the potential at each point, it is possible to obtain accurate vibrational spectra. By exploiting the locality of molecular vibrations, this method is applicable to the evaluation of spectra systems of moderate size. The judicious choice of Gaussian width parameters as well as selection augmentation of the basis with appropriate harmonic basis functions can lead to high-fidelity spectra even for relatively short trajectories.
This method provides a complement to perturbative approaches, as it treats low-frequency vibrations accurately and is amenable to systems with multiple low-lying energetic minima. Furthermore, by running more AIMD trajectories, it is possible to refine the vibrational spectrum obtained
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